Heretofore, so-called "coating type" magnetic recording media which are formed by applying powders of magnetic material dissolved in organic binders onto non-magnetic substrates and drying the same, have been generally used as recording media. However, the recent requirements of high-density magnetic recording have directed attention to so-called "metal thin film type" magnetic recording media having a large quantity of saturation magnetic flux. Research and development of magnetic media, such as an evaporated tape prepared by using an oblique evaporating method with Co, Fe, Ni and the like as a material and a perpendicular magnetic tape, disc and the like prepared by using a sputtering or evaporating method with a Co-Cr alloy, has been in progress.
Generally, the "metal thin film type" magnetic recording media have relatively large latent faculties in both electromagnetic transducing characteristics and recording density, compared to the latent faculties of the conventional "coating type" magnetic recording media. However, low weather resistance is a fatal weak point of the "metal thin film type" magnetic recording media.
As an example, when the media are left under an atmosphere of high temperature and humidity, oxidation of the metal film proceeds to cause a problem in that the gross amount of magnetic flux is decreased by 10% or more.
As another example, repetition of a dew condensation-drying process causes a problem in that rust is produced in an amount sufficient to see it with the naked eye.
As a further example, which relates to the so-called evaporated tape formed by applying cobalt onto a polyester film by the oblique evaporating method, rust is produced even in the case where the tape is left for about a week under an atmosphere of 60.degree. C. and 90% RH, the result being that the rust blocks the head in recording or reproducing operations in a deck. Furthermore, if a very small amount of NaCl fine crystals in the form of a sol, and about 1 ppm sulfurous acid gas or the like is present in the atmosphere, the rust increases to generate a serious barrier to use.
These phenomena of decreasing the magnetic flux and producing rust are generically called "weather resistance".
The problem of failure in weather resistance of the metal thin film type media has been generally recognized. Therefore, various techniques for improving weather resistance have been proposed. However, these techniques have the following problems: most of the techniques do not have a sufficient effect; some of the techniques sacrifice productivity for the necessity of large-scale equipment; and some of the techniques sacrifice other characteristics, such as electromagnetic transducing characteristics and the like. Hence, a more simple and more effective improvement in weather resistance has been desired.